1. Field of the Invention
The present invention relates to a battery charger and a charging method for charging a battery. The present invention relates, in particular, to a battery charger and a charging method suitable for charging a battery, such as a nickel metal hydride battery, which generates a large amount of heat during charging.
2. Description of the Related Art
Rechargeable battery have been recently used as a power source for a power tool, for example. A nickel metal hydride battery is widely used as the power tool battery of this type. To charge the battery, a battery charger for rapidly charging the battery with high current is used. That is, while one battery is being rapidly charged for about 20 minutes the, a power tool can be continuously used by replacing the currently used battery with another one which has been charged.
The inventor of the present invention studied ways to improve the performance of a power tool by using, as a battery, a nickel metal hydride battery. Although the nickel metal hydride battery can provide higher capacity than a nickel-cadmium battery, a large amount of heat is generated during charging. If the battery temperature becomes high due to the generated heat, the electrodes and separators of cells within the battery deteriorate and battery life is shortened. Due to this, the nickel metal hydride battery cannot be rapidly charged with high current using known charging techniques designed for a the nickel-cadmium battery.
Furthermore, the nickel metal hydride battery is more sensitive to overcharge than the nickel-cadmium battery and overcharging makes the battery life shorter. For that reason, it is required to avoid overcharging the nickel metal hydride battery. As for equipment which does not require battery replacement, to avoid overcharging the battery, charging current and discharging current are integrated and a battery is charged based on the integrated value. Thus, the battery can be fully charged. In case of charging a battery for equipment such as the above-stated power tool which requires replacing one battery with another, however, it has been difficult to fully charge the battery without overcharging it.
In one aspect of the present teachings, battery chargers and battery charging methods are taught that permit rapid and complete charging of the battery without overheating it. In one embodiment, a battery charger is taught that includes:
a memory for storing a map for mapping an allowable value of current, with which a battery can be charged while suppressing battery temperature from rising, based on a battery temperature and a battery temperature rise;
a temperature detection device for detecting a current temperature of the battery;
a temperature rise output device for obtaining the temperature rise from the temperature detected by the temperature detection device;
an allowable current retrieval device for retrieving the map of the memory device from the temperature detected by the temperature detection device and the temperature rise outputted from the temperature rise output device, and for obtaining the allowable value; and
a charging device for charging the battery with the allowable current retrieved by the allowable current retrieval device.
In another embodiment, a battery charger is taught that includes:
a memory for storing a map for mapping an allowable value of current, with which a battery can be charged while suppressing battery temperature from rising, based on a battery temperature and a battery-temperature rise, the map having the allowable current value set low when the battery temperature is high and set low when the temperature rise is large;
a temperature detection device for detecting a current temperature of the battery;
a temperature rise output device for obtaining the temperature rise from the temperature detected by the temperature detection device;
an allowable current retrieval device for retrieving the map of the memory device from the temperature detected by the temperature detection device and the temperature rise outputted from the temperature rise output device, and for obtaining the allowable value;
a charging device for charging the battery with the allowable current retrieved by the allowable current retrieval device;
a charging completion determination device for determining that charging is completed based on whether or not the temperature detected by the temperature detection device and the temperature rise outputted from the temperature rise output device belong to a region indicating a final charging stage (a region in which temperature rise is relatively large and relatively low allowable current is outputted) on the map of the memory device with high frequency; and
a charging completion device for completing charging based on the determination of the charging completion determination device that charging is completed.
In another embodiment, a charging method is taught that includes:
detecting current temperature of a battery;
obtaining a temperature rise from the detected temperature;
retrieving an allowable current map based on battery temperature and battery temperature rise, from the detected temperature and the obtained temperature rise, and obtaining an allowable value of current, with which the battery can be charged while suppressing the battery temperature from rising; and
charging the battery with the retrieved allowable current.
In another embodiment, a charging method is taught that includes:
detecting a current temperature of a battery;
obtaining a temperature rise from the detected temperature;
retrieving an allowable current map based on battery temperature and battery temperature rise, from the detected temperature and the obtained temperature rise, and obtaining an allowable value of current with which the battery can be charged while suppressing the battery temperature from rising;
charging the battery with the retrieved allowable current;
determining that charging is completed based on whether the outputted temperature rise is relatively large and whether relatively low allowable current is outputted from the map with a high frequency; and
completing charging based on the determination that charging is completed.
According to the battery charger and charging method described above, charging current is controlled using a map for mapping an allowable value of current with which a battery can be charged while suppressing battery temperature from rising based on battery temperature and battery temperature rise. That is, the map is retrieved from the battery temperature and temperature rise. The allowable value of current, with which the battery can be charged while suppressing the battery temperature from rising, is obtained. The battery is charged with the allowable current thus obtained. This makes it possible to charge a nickel metal hydride battery, the temperature of which tends to rise during charging, for a short period of time without causing deterioration due to temperature rise. Just before charging is completed, the temperature rise of the nickel metal hydride battery is larger and the battery is charged with relatively low current, therefore it is possible to suppress xe2x80x9covershootxe2x80x9d after the completion of charging.
In another aspect of the project teachings, the completion of charging is determined, in particular, based on whether or not a temperature rise is relatively large and whether or not relatively low current is outputted from the map with high frequency. That is, based on whether or not temperature rise is large and whether or not the rise is large even if charging current is decreased. Due to this, it is possible to fully charge the battery without fear of overcharging and without being influenced by the remaining battery capacity, temperature or other battery conditions.